To drink a potion, you must first enter the Meditate menu (not available during combat) and select the Drink Potion option. You can then select up to 3 potions to consume (4 if the Taster skill has been activated). The toxicity of selected potions cannot exceed 100 (the toxicity bar will turn orange and the potions will not be drunk). After exiting the meditation menu, the potions are immediately in effect. They are best to be consumed just before a fight.

For a detailed list of potion effects and the impact of the Catalysis skill. please see Effects of Catalysis.